The present invention relates to a fixing device which fixes a toner image on a recording material and to an image forming apparatus having the fixing device to form an image, such as a coping machine, a printer and a facsimile machine.
The image forming apparatus such as the coping machine, the printer, and the facsimile machine utilizing an electro photographic method forms an image on the recording material by transferring a toner image from a surface of an image carrier such as a photoreceptor and an intermediate transfer member and by fixing the transferred toner image.
In a fixing device commonly used for in the image forming apparatus, a nip area is formed by 2 rollers including a heat roller and a pressure roller, and the recording material is caused to pass through the nip area to be fixed. In the nip area in the fixing device, there has been occurred a problem that the recording material enwinds the heat roller. To solve the problem, a technology to cause separating claws to make contact with a heat roller surface is proposed.
The claw is effective to prevent enwinding, however the claw makes streak line scratches on the heat roller surface. These scratches on the heat roller appear on the image of the recording material as defects, which has been a problem for image forming apparatuses which are required to have high quality. As a technology considering this point of view, there is proposed a separation claw moving device by which the separation claws are able to move in a direction parallel to a rotation axis of the heat roller and may reciprocate in a claw moving area at a constant velocity.
[Patent Document 1] Japanese Patent Publication Laid-Open No. HEI 9-160390
However, though the separation claws move in the direction parallel to the rotation axis of the heat roller, if the moving velocity of the separation claw (a separation claw's moving distance along the surface of heat roller per unit time) is low, the scratches which are sufficient to generate the defects in the image on the recording material will occur on the heat roller surface.
Also, if the moving velocity of separation claw is high, a problem occurs in performance of separation. This aspect is explained using FIG. 7. FIG. 7 is a drawing showing a status where the separation claws move along the surface of the heat roller. Numeral 1000 is the separation claw and numeral 1001 is the heat roller. The separation claws reciprocates along the surface of heat roller. If the moving velocity of separation claw 1000 is high, there is occurred a phenomenon that one side of separation claw 1000 is lifted (hereinafter called bias contact) as shown in FIG. 7. There is no problems in separation performance as long as separation claw 1000 is in contact with heat roller 1001 as shown in FIG. 7(a), however in bias contact condition as shown in FIG. 7(b), the separation claw cannot make contact sufficiently with the recording material adhering to the heat roller sufficiently, and preferable performance of separation cannot be exerted.
Therefore, an object of the present invention is to provide a fixing device and an image forming apparatus which prevents the defects of the image and the problem of separation performance of the separation claw, by focusing attention on the moving velocity of the separation claw.